Solid-state image sensor with an optical black area having pixels for detecting black level

ABSTRACT

A digital camera includes an image sensor having an array of CCD (Charge-Coupled Device) cells on which an optical image is focused via a lens and a shutter. The image sensor produces an image signal by photoelectric conversion. The image signal is clamped by an analog front end circuit, which feeds a processed analog signal to an analog-to-digital converter. The resultant digital signal is input to a signal processor. The analog front end circuit switches horizontal scanning lines to be clamped in accordance with image pickup information. The horizontal scanning lines to be clamped lie in an optical black area where a photodiode is included in each CCD cell or an optical black area where the former is not included in the latter.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a solid-state image sensor and an imagepickup apparatus including the same. More particularly, the presentinvention relates to a solid-state image sensor capable of enhancingimage quality by stabilizing a signal level, e.g. a black level, andcapable of accurately correcting the black level when mounted on adigital camera, scanner or similar image input apparatus, and to animage pickup apparatus including the same.

2. Description of the Background Art

A digital camera, for example, is configured to pickup a desired subjectby focusing the optical image of the subject on a solid-state imagesensor. The current trend in the imaging art is toward a solid-stateimage sensor having pixels densely arranged thereon for improving thequality of an image picked up thereby. Japanese patent laid-openpublication No. 2002-290841, for example, discloses an image pickupapparatus including an effective pixel area, or image pickup area, and aplurality of optical black (OB) areas. The image pickup apparatusdisclosed is constructed such that, when intense light is incident onone optical black area, an accurate reference black-level signal isobtained from the remaining optical black areas in order to reduceblackout.

Japanese patent No. 2806035, for example, teaches a solid-state imagesensor including an effective pixel area and an optical black areasurrounding it. In this case, when the image of a subject picked up bythe image sensor is to be reproduced from image data representative ofthe image, a black level is corrected by use of data output from theoptical black area. The above patent further teaches a video signalprocessing circuit for controlling the potential variation of a videosignal during black-level periods.

However, the prior art schemes stated above have a problem that aplurality of optical black areas must be arranged above and below theeffective pixel area in the imaging plane, and a problem that controlover switching of the optical black areas and other sophisticatedcontrol processing are required. Consequently, it is difficult tofurther enhance image quality in a high-sensitivity pickup mode, along-exposure pickup-mode or similar image pickup mode.

A conventional optical black area is implemented by a shield layerformed on the top of the individual photoelectric transducer orphotosensitive cell. This, however, gives rise to a problem that, whenthe level of an output signal from the optical black area varies due totemperature elevation or extremely long exposure time and rises relativeto the black-level output of the effective pixel area, the variation ofa black signal output from the effective pickup area is cut off by theblack level of the optical black area, rendering dark regionspractically “flat black”. Once the variation of black gradation pickedup by the effective pixel area is cut off by the output level of theoptical black area, then the variation of the black gradation cannot berestored. It is therefore necessary to correct the black level with highaccuracy.

On the other hand, when temperature is low or when exposure time isextremely short, the black level output from the effective pixel arearises relative to the black level of the optical black area, renderingdark regions light. This problem, however, can be coped with by signalprocessing and therefore not so critical.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide a solid-state imagesensor capable of further enhancing image quality and correcting theblack level with high accuracy, and an image pickup apparatus using thesame.

A solid-state image sensor in accordance with the present inventionincludes an effective pixel area which has a plurality of photosensitivecells arranged for each generating a signal charge corresponding to anamount of incident light, and an optical black area shielded from theincident light. The optical black area is positioned at the front andrear sides of horizontal scanning lines. The horizontal scanning linesinclude first horizontal scanning lines, including first zones in whichthe photosensitive cells are present, and second horizontal scanninglines, including second zones in which the photosensitive cells areabsent, arranged alternately with each other at the front side atpredetermined intervals each.

Also, an image pickup apparatus for picking up a field to generate animage signal representative of the field in accordance with the presentinvention includes a solid-state image sensor which includes aneffective pixel area having a plurality of photosensitive cells forgenerating a respective signal charge corresponding to an amount ofincident light each, and an optical black area shielded from theincident light. The optical black area is positioned at the front andrear sides of horizontal scanning lines. The horizontal scanning linesinclude first horizontal scanning lines, including first zones in whichthe photosensitive cells are present, and second horizontal scanninglines, including second zones in which the photosensitive cells areabsent, arranged alternately with each other at the front side atpredetermined intervals each. The solid-state image sensor is driven bya driver to output an image signal in accordance with exposure. Ananalog circuit clamps the image signal output from the solid-state imagesensor while a controller controls clamping executed by the analogcircuit. The controller causes the analog circuit to selectively clampthe first horizontal scanning lines or the second horizontal scanninglines.

Further, a solid-state image pickup apparatus for picking up a field togenerate an image signal representative of the field in accordance withthe present invention includes an effective pixel area having effectivepixels and an optical black area surrounding it and having pixels. Aplurality of kinds of pixels different in structure from each other arearranged in the optical black region for detecting a black level of theimage signal.

BRIEF DESCRIPTION OF THE DRAWINGS

The objects and features of the present invention will become moreapparent from consideration of the following detailed description takenin conjunction with the accompanying drawings in which:

FIG. 1 is a schematic block diagram showing a preferred embodiment of animage pickup apparatus in accordance with the present invention;

FIG. 2 is a plan view schematically showing a specific configuration ofa solid-state image sensor included in the illustrative embodiment shownin FIG. 1 and including an effective pixel area, an optical black areaand zones with photodiodes and zones without photodiodes alternatelyarranged in the optical black area;

FIG. 3 is a vertical section showing one of a number of cells arrangedin the effective pixel area;

FIG. 4 shows a vertical section of one of the photosensitive cellspositioned in any one of the zones with photodiodes included in theoptical black area;

FIG. 5 shows a vertical section of one of the photosensitive cellspositioned in any one of the zones without photodiodes also included inthe optical black area;

FIG. 6 is a view, similar to FIG. 2, schematically showing analternative specific configuration of the solid-state image sensor inwhich the zones with photodiodes and zones without photodiodes arearranged in the rear portion of the optical black area also;

FIG. 7 is a timing chart useful for understanding a specific clampingoperation effected by clamp pulses;

FIG. 8 is a plan view schematically showing an alternative embodiment ofthe image pickup apparatus in accordance with the present invention;

FIG. 9 shows the output level of a signal output from an optical blackarea and that of a signal output from an effective pixel area includedin a conventional solid-state image sensor;

FIG. 10 is a plan view schematically showing two of a number of pixelsarranged in a solid-state image sensor included in the alternativeembodiment;

FIG. 11 is a sectional perspective view showing a specific configurationof one of the photosensitive cells positioned in an effective pixel areaincluded in the image sensor of FIG. 10;

FIG. 12 is a view, similar to FIG. 11, showing a specific configurationof one of the photosensitive cells positioned in an optical black areaalso included in the image sensor of FIG. 10;

FIG. 13 is a sectional perspective view showing an alternative specificconfiguration of one of the photosensitive cells positioned in theoptical black area of FIG. 10;

FIG. 14 is a sectional perspective view showing a still alternativespecific configuration of one of the photosensitive cells positioned inthe optical black area of FIG. 10;

FIG. 15 is a sectional perspective view showing a further specificconfiguration of one of the photosensitive cells positioned in theoptical black area of FIG. 10; and

FIG. 16 shows an output level of a signal output from the effectivepickup area and that of a signal output from the optical black areaincluded in the alternative embodiment.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring now to FIG. 1 of the accompanying drawings, an image pickupapparatus to which the present invention is applied is implemented as adigital camera by way of example. As shown, the digital camera,generally 10, includes a solid-state image sensor 16. When an opticalshutter 14 is opened, light representative of a subject field isincident on the image sensor 16 via a lens system 12 and converted to anelectric pixel signal 38 thereby.

An analog front end (AFE) processor 18 performs correlated doublesampling (CDS) and clamping with the pixel signal 38 output from theimage sensor 16, and feeds the resulting analog signal 60 to ananalog-to-digital (AD) converter 20. The AD converter 20 converts theanalog signal 60 to corresponding digital image data and delivers theimage data 62 to a signal processor 22. The signal processor 22 writesthe image data 64 in an image memory 24 for thereby preparing image datato be displayed and recorded. Such image data are also output to amonitor 26 and a recorder 28 as depicted with connections 66 and 68,respectively. The image pickup processing described above is executedunder the control of a controller 30 in response to a command signal 70output from a control panel 32 connected to the controller 30. In theillustrative embodiment, the controller 30 is implemented by amicrocomputer including a CPU (Central Processing Unit). The recorder 28is loaded with a semiconductor memory or similar data recording mediumfor storing the image data 64 therein.

The solid-state image sensor 16, positioned on the focusing plane, notshown, of the lens system 12, may advantageously be implemented by acolor charge-coupled device (CCD) image sensor of interline transfer(IT) type. A number of photodiodes (PDs), not shown, are arranged on theimage pickup surface of the image sensor 16 to form an array ofphotosensitive cells, which transform incident light 84 to acorresponding signal charge each. The image sensor 16 is driven with adrive signal 36 provided by a driver 34. The drive signal 36 includes avertical and a horizontal transfer clock φV and φH, respectively, and anoverflow drain (OFD) voltage and other various voltages, in accordancewith the operation mode and read-out control, so that the signal chargesstored in the photodiodes are read out as pixel signals. The pixelsignals thus produced are fed to the analog front end circuit 18 fromthe output 38 of the image sensor 16.

More specifically, the driver 34 generates the drive signal 36 inresponse to signals 72 including H (horizontal) and V (vertical) pulsesand OFD pulses fed from a timing generator 40. The timing generator 40further feeds a control signal 74 to the lens system 12 for controllingthe focal position of the lens system 12, and feeds another controlsignal 76 to the optical shutter 14 for selectively opening or closingit.

FIG. 2 schematically shows a specific arrangement of the image sensor 16in a plan view. The image sensor 16 includes photodiodes orphotosensitive cells arranged in bidimensional arrays, forming aphotosensitive array or light incidence surface, although not shownspecifically. As shown in FIG. 2, the image sensor 16 is made up of aneffective pixel area or pickup area 200 and an optical black (OB) area202 which is optically shielded from light. The photodiodes are arrangedin the effective pixel area 200 such that signal charges generatedtherein can be read out.

In the front portion of the optical black area 202 positioned at theleft-hand side of the effective pixel area 200 in the figure, zones withphotodiodes 210 and zones without photodiodes 211 are arrangedalternately with each other and are positioned at the interval ofintegral number of the horizontal lines each. More specifically, thezones with photodiodes 210 are arranged on even-numbered lines n, n+2and so forth while the zones without photodiodes 211 are arranged onodd-numbered lines n+1, n+3 and so forth. The zones with photodiodes 210and zones without photodiodes 211 should preferably be arranged at theinterval of a horizontal scanning line or at a preselected intervaleach.

In the illustrative embodiment, the photodiodes or photosensitive cellsin the front part of the optical black area 202 are arranged on theimage pickup surface of the image sensor 16 in a honeycomb pattern, i.e.shifted from each other by one-half of a pixel pitch in the horizontaland vertical directions. If desired, the zones with photodiodes andzones without photodiodes may be located in the rear portion of theoptical black area 202.

FIG. 3 shows a vertical section having a specific structure of any oneof the photosensitive cells 200 arranged in the effective pixel area200. As shown, the effective pixel area 200 includes an N-typesemiconductor substrate 300 on which a P-type potential well 302 isformed. An N⁺ layer 306 and a P⁺ layer 308 are formed in this order onthe portion of the P-type well 302 corresponding to an optical aperture304, which will be described later, forming a photosensitive regionimplemented as a photodiode. P-type channel stoppers 310 are formed atopposite sides of the photodiode while a buried channel 312, functioningas a vertical transfer register, are positioned at the outside of thechannel stopper 310 and formed of an N-type semiconductor. Above theburied channel 312, a transfer electrode 314 is positioned.

Further, an optical shield layer 320 for intercepting incident lightbeams 84 covers the transfer electrode 314 and is formed with theaperture 304 mentioned earlier that allows incident light to reach thephotodiode. The N⁺ layer 306 and P⁺ layer 308 are positioned beneath theaperture 304. A color filter segment 330 is formed above the aperture304 while a microlens 340 is formed above the color filter segment 330.The effective pixel area 200 includes a number of photosensitive cells20 structured as described above.

FIG. 4 illustrates a vertical section of a specific structure of one ofthe photosensitive cells 210 included in one of the zones 210 withphotodiodes. As shown, the cell of the zone with photodiodes 210 may bethe same as the cell of the effective pixel area 200, FIG. 3, exceptthat the shield layer 320 is extended to form an optical shield layer400 that covers not only the transfer electrodes but also thephotodiode. The remaining structural elements of the zone withphotodiodes 210 are identical with the structural elements of theeffective pixel area 200 and will not be described specifically in orderto avoid redundancy. In the figures, of course, like components aredesignated with the same reference numerals.

FIG. 5 shows, in a vertical sectional view, one of the cells in thezones 211 without photodiodes. As shown, the zone 211 withoutphotodiodes may be the same as the zone 210 with photodiodes except thatthe N⁺ layer 306 and P⁺ layer 308, constituting a photodiode, areabsent. The remaining structural elements of the zone withoutphotodiodes 211 are identical with the structural elements of the zonewith photodiodes 210, FIG. 4, and will not be described specifically inorder to avoid redundancy.

Referring again to FIG. 1, the analog front end circuit (AFE) 18,connected to the output 38 of the image sensor 16, is an analog signalprocessing circuit configured to execute CDS sampling and automatic gaincontrol (AGC) on the input pixel signal 38. Specifically, the analogfront end circuit 18 includes a clamping circuit, not shown, configuredto clamp the pixel signal 38 in order to hold the black level of thepixel signal and correct the black level. The output 60 of the analogfront end circuit 18 is connected to the signal processor 22 via the ADconverter 20. In the illustrative embodiment, the signal processor 22 isadapted to transform the color image signal 62, containing colorcomponent signals, to the signals 64, 66 and 68 including a luminancesignal and a chrominance signal representative of a luminance and acolor difference, respectively, in response to timing signals 78including a vertical and a horizontal synchronous signal VD and HD,respectively, and an operational clock signal CLK fed from the timinggenerator 40. The signal processor 18 feeds the luminance signal to thecontroller 30 over a connection 80, so that the controller 30 candetermine an exposure value in dependent upon the image signal andcontrol the pickup processing.

The signal processor 22 provides the monitor 26 and recorder 28 withimage signals thus produced through the digital signal processing. Therecorder 28 serves as recording the image data 68 received from thesignal processor 22 in the data recording medium loaded thereon. In theillustrative embodiment, the recorder 28 includes a compressing circuit,not shown, for coding the image data by compression.

The controller 30 controls various sections of the camera 10 in responseto various command signals 70, including a shutter release signal, fedfrom the control panel 32. Particularly, the controller 30 controls theimage sensitivity of the camera 10 in accordance with sensitivityselected on the control panel 32 by the operator, and is capable ofautomatically setting a shutter speed and an exposure value althoughthey may, of course, be set by the operator. Further, the controller 30controls pixel interpolation on the basis of defective-pixel data storedin a defective-pixel store 50, which may be implemented by a read-onlymemory (ROM).

Moreover, the controller 30 has a function of switching, when along-term exposure, or slow shatter, mode is elected by the operator,control over the analog front end circuit 18 via the timing generator40. More specifically, when an exposure time longer than a preselectedperiod of time is selected, the controller 30 causes the analog frontend circuit 18 to clamp the portions of the image signal 38representative of the zones without photodiodes 211, FIG. 2, of theoptical black area 202. At this instant, the timing generator 40, underthe control of the controller 30, feeds the analog front end circuit 18with clamp pulses 82 that clamp the signals originating from the zoneswithout photodiodes 211, but do not clamp the signals from the zoneswith photodiodes 210. This is successful in implementing a referencelevel in a condition wherein the adverse influence of dark currentsgenerated in the photodiodes during long-exposure in a vertical scanningperiod. In addition, when the image data should be produced, thecontroller 30 is capable of preparing an image file including the imagedata and pickup information representative of horizontal scanning lineson which clamping was effected.

As stated above, in the illustrative embodiment, the zones withphotodiodes 210 and zones without photodiodes 211 are arranged at thepredetermined intervals each in the front portion of the optical blackarea 202 as if they were lines alternating with each other. It istherefore possible to effect clamping at the timing of signals read outfrom the zones without photodiodes 211 and free from the adverseinfluence of dark currents generated in the photodiodes in, e.g. thelong-exposure mode, thereby insuring a reference signal with a stableblack level.

Now, with reference to FIG. 6, described will be an alternative specificarrangement of the image sensor 16. As shown, the image sensor, labeled600, includes an effective pixel area 200 and an optical black area 202like the image sensor 16 of FIG. 2. Zones with photodiodes 611 and zoneswithout photodiodes 610 are arranged in the rear optical black portion602 positioned at the right-hand side, in the figure, of the effectivepixel area 200 alternately with each other.

More specifically, the zones with photodiodes 610 are arranged onodd-numbered lines n+1, n+3 and so forth while the zones withoutphotodiodes 611 are arranged on even-numbered lines n, n+2 and so forth.In this manner, the zones without photodiodes 611 formed in the rearpart of the optical black area 600 are positioned on the same horizontallines as the zones with photodiodes 210 formed in the front part of theoptical black area 202, FIG. 2, while the zones with photodiodes 610 arepositioned on the same lines as the zones without photodiodes 211. Asfor the rest of the configuration, the image sensor 600 may be identicalwith the image sensor 16.

Reference will be made to FIG. 7 for describing control over the analogfront end circuit 18 to be executed when the image sensor 600 of FIG. 6is substituted for the image sensor 16 of FIG. 2. As shown, when theimage signal is to be clamped by clamp pulses CLPa generated in theevent of usual clamping, a pulse signal for clamping part of the imagesignal corresponding to the rear part of the optical black area 202 isfed to the analog front end circuit 18, FIG. 2. In this case, becausenot all lines are clamped, other clamp pulses CLPb are fed to the analogfront end circuit 18 in accordance with the high-sensitivity mode,long-exposure mode or similar mode selected by the operator.

In the high-sensitivity mode or the long-exposure mode, for example,part of the image signal corresponding to the front part of the opticalblack area 202 is clamped at the same time as part of the image signalcorresponding to the rear part of the same is clamped by the clamppulses CLPa, so that the clamp potential at the front side isstabilized.

Generally, in an image sensor, dark currents generated in verticaltransfer paths or VCCDs (Vertical CCDs) are greater than dark currentsgenerated in photodiodes. More specifically, dark currents generated inthe VCCDs during the transfer of signal charges are far greater thandark currents generated in photodiodes. In the long-exposure mode, forexample, dark currents generated in photodiodes increase during exposureeffected at a shutter speed that extends a storage time in thephotodiodes, resulting in local irregularity in dark current andtherefore noise in shadow regions in a reproduced picture. Thus, byswitching horizontal scanning lines to be clamped in accordance with thepickup mode, it is possible to prevent, e.g. dark regions from beingrendered practically flat black due to the analog clamping.

More specifically, in the long-exposure mode exceeding a preselectedperiod of time, the controller 30 causes the analog front end circuit 18to clamp the lines of the zones without photodiodes because the clamppotential is apt to rise on the lines of the zones with photodiodes dueto white noise or similar defect. On the other hand, in thehigh-sensitivity mode, the controller 30 causes the analog front endcircuit 18 to clamp signals on the lines of the zones with photodiodesbecause the image signal to be dealt with falls in level and influencesthe image quality of shadow regions due to small differences in level.

The control over clamp switching stated above is effective even when,e.g. a processed image signal is recorded in a recording medium in theraw format, i.e. in the form of R (red), G (green) and B (blue) signals,by switching control in accordance with pickup informationrepresentative of, e.g. sensitivity selected or long exposure.

As stated above, in the configuration shown in FIG. 6, the zones withphotodiodes 610 and zones without photodiodes 611 are arranged in therear optical black portion 602 alternately with each other, and each isspaced by an integer number of lines. With this configuration, theillustrative embodiment is free from the influence of light accidentallytransmitted through the optical shield layer of the optical black areaand the influence of dark currents generated in the photodiodes in thelong-exposure mode for thereby insuring accurate correction of the blacklevel.

Further, because the front optical black portion and rear optical blackportion are opposite to each other as to the relation between the zoneswith photodiodes and zones without photodiodes, the illustrativeembodiment obviates omission otherwise occurring due to the clamping ofonly the rear optical black portion, and enhances potential stability byeffecting clamping in the front optical black portion.

Moreover, by switching the lines to be clamped in dependence upon theexposure time, subjective luminance information, ISO (InternationalStandards Organization) sensitivity or similar pickup information, theillustrative embodiment obviates the influence of, e.g. dark regionsrendered practically black. In this case, horizontal scanning lines fromwhich black-level correction data should be produced on the basis of thepickup information may be determined in accordance with the control overthe switching of clamp lines. If desired, the clamp-line switchingcontrol and clamped lines may be recorded as pickup information, so thatpositions where the subtraction of the black level should be effectedcan be determined in accordance with the above-stated lines in the eventof image processing to be executed later. This is desirable in the casewhere raw-data file output is to be processed.

An alternative embodiment of the present invention will be describedhereinafter. FIG. 8 is a plan view showing a solid-state image sensor ofthe alternative embodiment. As shown, the image sensor illustrated isgenerally made up of an effective pixel area 800 and an optical blackarea 801 surrounding it. A number of photosensitive cells are arrangedin each of the effective pixel area 800 and optical black area 801 in amatrix. The photosensitive deices, i.e. photodiodes in the optical blackarea 801 each are optically shielded from incident light by a respectiveshield layer.

FIG. 9 shows output levels of the image sensor to appear in a particularcondition in lines (a) (b) and (c), as will be described specificallylater. Dummy pixels, not shown in FIG. 8, are arranged in the mostperipheral portion of the image sensor surrounding the optical blackarea 801 while the effective pixel area 800 is formed inside of theoptical black area 801. A signal derived from dark currents is outputfrom the optical black area 801 while image data representative of asubject picked up are output from the effective pixel area 800.

FIG. 9, line (a), shows ideal output levels of the image sensor toappear when the entire photosensitive array or surface of the imagesenor is shield from incident light. As understood from the figure, ablack level output from the optical black area 801 and a black leveloutput from the effective pixel area 800 are coincident with each other.In practice, however, the output level of the optical black area 801varies in dependence upon, e.g. temperature. For example, as seen fromFIG. 9, line (b), when the black level output from the optical blackarea 801 rises relative to the black level output from the effectivepixel area 800 due to high temperature or extremely long exposure time,changes in the black level output from the effective pixel area 800 arepractically cut off by the black level of the optical black portion 801,rendering dark regions entirely flat black.

On the other hand, as shown in FIG. 9, line (c), when the black leveloutput from the effective pixel area 800 rises relative to the blacklevel output from the optical black area 801 due to low temperature orextremely short exposure time, dark regions are unexpectedly renderedlight. This problem, however, can be coped with by signal processing andis therefore not critical, as stated earlier.

FIG. 10 is a plan view showing a specific configuration of aphotosensitive cell or pixel formed in each of the effective pixel area800 and optical black area 801 included in the illustrative embodiment,and also implemented by an IT-CCD device mentioned previously. WhileFIG. 10 shows only two cells or pixels adjoining each other in thevertical direction, a number of such cells are of course arranged inboth of the vertical and horizontal directions. As shown, each cell hasa substantially square shape in which a photodiode or photoelectrictransducer 100 and a vertical transfer path 102 are formed at theleft-hand side and right-hand side, as viewed in FIG. 10, respectively.

FIG. 11 is a sectional perspective view cut along a line X-X of FIG. 10showing one of the cells, labeled 120, arranged in the effective pixelarea 800, FIG. 8, of the alternative embodiment. As shown, the cell 120includes an N-type semiconductor substrate 105 formed with a P-welllayer 106. An N⁺ layer 107 and a P⁺ layer 108 are sequentially stackedon the top of the P-well layer 106 in this order, constituting thephotodiode 100, FIG. 10.

An N-type vertical transfer path 102 a is formed at the right-hand side,as viewed in FIG. 11, of the photodiode 100 while a P-type read-out gate109 for reading out a signal charge is formed between the verticaltransfer path 102 a and the photodiode 100. A P-type channel stopper 110is formed at the left-hand side, as viewed in FIG. 11, of the photodiode100 remote from the vertical transfer path 102 a, potentially isolatingthe vertical transfer path 102 a from an N-type vertical transfer path102 b assigned to the adjoining pixel.

Transfer electrode layers 111 a and 111 b are formed on the verticaltransfer paths 102 a and 102 b, respectively. An optical shield layer112 is positioned above the transfer electrode layers 111 a and 111 band formed with an optical aperture 112 a above the photodiode 100. Withthe aperture 112 a, the shield layer 112 allows light to be incident onthe photodiode 100 while shielding the vertical transfer paths 102 a and102 b from the light. A color filter 113 is stacked on the shield layer112 while a microlens 114 is formed above the color filter 113.

In the configuration shown in FIG. 11, part component of the lightincident on the microlens 114 identical in color with the color filter113 is transmitted through the color filter 113 and then passed throughthe aperture 112 a of the shield layer 112 to the photodiode 100. As aresult, a signal charge corresponding to the amount or intensity ofincident light component is generated and stored in the photodiode 100.Subsequently, when a read pulse is applied to a read-out electrode, notshown, the signal charge is read out from the photodiode 100 to thevertical transfer path 102 via the read-out gate.

Thereafter, when a transfer pulse is applied to the transfer electrodelayer 111 a (φ1, φ2, φ3 or φ4, FIG. 10), the signal charge istransferred to a horizontal transfer path, not shown, via the verticaltransfer path 102 (102 a or 102 b). The signal charge is then outputfrom the image sensor via the horizontal transfer path.

FIG. 12 is a sectional perspective view along the line X-X in FIG. 10showing a specific configuration of one of the cells or pixels arrangedin the optical black area 801, FIG. 8. As shown, the cell, labeled 130,is identical with the cell 120 of FIG. 11 except that the shield layer112 is not formed with the aperture 112 a, FIG. 11, but is configured tooptically shield even the photodiode 100 from incident light. In FIG.12, structural parts and elements like those shown in FIG. 11 aredesignated by identical reference numerals, and a detailed descriptionthereof will not be made in order to avoid redundancy. In thisconfiguration, the image data or signal charge output from the cell 130are representative of black and are also sequentially transferred viathe vertical and horizontal transfer paths and then output from theimage sensor.

FIG. 13 is a sectional perspective view along the line X-X of FIG. 10showing an alternative specific configuration of the cell positioned inthe optical black area 801, FIG. 8. As shown, the cell, labeled 140, isidentical with the cell 120 of FIG. 11 in that the shield layer 112 isformed with the aperture 112 a, but different from the cell 120 in thata red (R) filter 113R, a green (G) filter 113G and a blue (B) filter113B are stacked on the shield layer 112 and in that the microlens 114is omitted in order to reduce the incidence efficiency of light. In FIG.13 also, structural parts and elements like those shown in FIG. 11 aredesignated by identical reference numerals, and a detailed descriptionthereof will not be made in order to avoid redundancy.

More specifically, the cell 140 is configured to cause the filters 113R,113G and 113B to ultimately absorb the entire incident light, therebypreventing the incident light from reaching the photodiode 100. In thisconfiguration, the image data or signal charge output from the cell 140is also representative of black and is also sequentially transferred viathe vertical and horizontal transfer paths and then output from theimage sensor.

The cell 130 shown in FIG. 12, positioned in the optical black area 801is configured to intercept incident light with the shield layer 112.Because the shield layer 112 is usually implemented by a metallic filmformed of, e.g. tungsten, light incident on the shield layer 112 isconsidered to cause a dark current generated in the photodiode 100, andhence the black level, to vary. This is why the cell 140 shown in FIG.13 absorbs the entire incident light with the color filters 113R, 113Gand 113B positioned above the aperture 112 a of the shield layer 112.

While the cell 140 of FIG. 13 includes a stack of three color filters113R, 113G and 113B, the stack may alternatively or additionally includea single color filter of black, if desired. Alternatively, the shieldlayer 112 of the cell 140 may be adapted to have no aperture 112 aformed in addition to the structure described above.

FIG. 14 is a sectional perspective view along the line X-X in FIG. 10showing a still alternative specific configuration of the cellpositioned in the optical black area 801, FIG. 8. As shown, the cell,labeled 150, is identical with the cell 130 of FIG. 12 in that theshield layer 112 is not formed with the aperture but intercepts theentire incident light. The cell 150 is, however, different from the cell130 in that the photodiode is omitted, or not formed, in order toobviate the variation of the dark current. In FIG. 14 also, structuralparts and elements like those shown in FIG. 12 are designated byidentical reference numerals, and a detailed description thereof will ofcourse not be made in order to avoid redundancy.

FIG. 15 is a sectional perspective view along the line X-X of FIG. 10showing a further specific configuration of the cell positioned in theoptical black area 801, FIG. 8. As shown, the cell, labeled 160, isidentical with the cell 130 of FIG. 12 except that the channel stopperis increased in width while the vertical transfer paths 102 a or 102 iscorrespondingly decreased in width. In FIG. 15, structural parts andelements like those shown in FIG. 12 are designated by the samereference numerals, and a detailed description thereof will not be madein order to avoid redundancy. The width of the vertical transfer path102 a or 102 b is reduced because a dark current generated in thevertical transfer path 102 a or 102 b is predominant over a dark currentgenerated in the photodiode 100.

It has been customary with a solid-state image sensor to arrange, amongthe cells or pixels 130, 140, 150 and 160 described above with referenceto FIGS. 12, 13, 14 and 15, respectively, only the cell 130 shown inFIG. 12 in the optical black area 801, FIG. 8. By contrast, thealternative embodiment allows at least two of the four different cells130 through 160 to be arranged in the optical black area 801.

For example, cells with the configuration shown in FIG. 12 and cellswith the configuration shown in FIG. 13 may be respectively arranged inthe left optical black portion 801L and right optical black portion 801Rof FIG. 8. Alternatively, cells with the configuration shown in FIG. 12and cells with the configuration shown in FIG. 13 may be arranged in theright optical black portion 801R in which about thirty pixels arepositioned side by side. Further, two of the four different kinds ofcells may be respectively arranged in the upper and lower portions ofthe optical black area 801. In any case, the cells positioned in theoptical black area 801 for correcting the black level of image dataoutput from the cells of the effective pixel area 800 are selectivelyused in the usual pickup mode or a pickup mode in which a dark currentis expected to increase.

More specifically, in the usual pickup mode, the output level of thecells with the configuration of FIG. 12 is used as a black level forcorrecting the black level of image data output from the effective pixelarea 800. On the other hand, when a dark current is expected to rise dueto, e.g. temperature higher than a predetermined value or sensitivityselected higher than a predetermined value, the output level of thecells with the configuration of FIG. 13 is used as a black level forcorrecting the black level of image data output from the effective pixelarea 800.

FIG. 16 shows black levels output from the image sensor of thealternative embodiment. As shown, assume that a black level a outputfrom each optical black cell 130 rises above the black level b of theeffective pixel area 800 due to an increase in dark current which is, inturn, ascribable to temperature elevation or similar cause. Even in sucha condition, a black level c output from each optical black cell 140,which is different in structure from the optical black cell 130, isdifferent from the black level a, increasing the possibility that theblack level of the effective pixel area 800 can be corrected by thelevel c on the basis of signal processing.

While the optical black cells 130 through 160 each having particular oneof four different structures have been shown and described, it cannot beestimated which of them reduces a dark current most under various pickupconditions. In light of this, in the alternative embodiment, opticalblack cells having a plurality of different configurations are used incombination. Alternatively, three, four or more kinds of optical blackcells may be arranged in combination, in which case optical black cellsoutputting the lowest black level are used for correcting the blacklevel output from the effective pixel area 800.

It is to be noted that the alternative embodiment is practicable notonly with a CCD image sensor shown and described but also with a CMOS(Complementary Metal Oxide Semiconductor) image sensor except for thestructure shown in FIG. 15 in which the width of the vertical transferpath is increased.

As stated above, the alternative embodiment selectively uses a pluralityof different kinds of cells or pixels arranged in the optical black areain combination in order to enhance accurate correction of the blacklevel of image data, implementing a solid-state image sensor to bemounted on a digital camera, scanner or similar image input apparatus.

In summary, in accordance with the present invention, zones withphotodiodes and zones without photodiodes are arranged in an opticalblack area alternately at predetermined intervals each, so that theinfluence of light having passed through a shield layer included in theoptical black area and that of dark currents generated in photodiodes ina long-exposure mode can be obviated. It follows that a black leveloutput from an effective pixel area can be accurately corrected. Also,the front and rear portions of the optical black area are opposite toeach other as to the positions of the zones with photodiodes and zoneswithout photodiodes. This successfully obviates omission otherwiseoccurring when only the rear optical black portion is clamped, i.e.enhances potential stability by clamping the front optical black portionalso.

Further, lines to be clamped in level are switched in accordance withpickup information in order to obviate the influence of, e.g. blackregions undesirably rendered practically black. In this case, horizontalscanning lines from which black-level correction data on the basis ofthe pickup information should be obtained can be determined inaccordance with control over the switching of the clamp lines. Theclamp-line switching control and clamped lines may be recorded in theform of image pickup information, so that positions where thesubtraction of the black level should be effected can be determined inaccordance with the above-stated lines in the event of image processingto be executed later. This is desirable in the case where a raw datafile is to be processed.

Moreover, a plurality of different kinds of cells or pixels arranged inthe optical black area in combination are selectively used in order toenhance accurate correction of the black level of image data.

The entire disclosure of Japanese patent application Nos. 2005-122035and 2005-162590 respectively filed on Apr. 20, 2005 and Jun. 2, 2005,including the specifications, claims, accompanying drawings andabstracts of the disclosure, are incorporated herein by reference in itsentirety.

While the present invention has been described with reference to theparticular illustrative embodiments, it is not to be restricted by theembodiments. It is to be appreciated that those skilled in the art canchange or modify the embodiments without departing from the scope andspirit of the present invention.

1. A solid-state image sensor comprising: an effective pixel area havinga plurality of photosensitive cells arranged, each of the plurality ofphotosensitive cells generating a particular signal charge correspondingto an amount of incident light; and an optical black area shielded fromthe incident light; wherein said optical black area is positioned at afront side and a rear side of horizontal scanning lines, the horizontalscanning lines comprising first horizontal scanning lines, includingfirst zones in which the photosensitive cells are present, and secondhorizontal scanning lines, including second zones in which thephotosensitive cells are absent, arranged alternately with each other atthe front side at predetermined intervals each.
 2. The image sensor inaccordance with claim 1, wherein said first scanning lines, includingthird zones in which the photosensitive cells are absent, and saidsecond scanning lines, including fourth zones in which thephotosensitive cells are present, are arranged in the rear side.
 3. Animage pickup apparatus for picking up a field to generate an imagesignal representative of the field, comprising: a solid-state imagesensor comprising an effective pixel area having a plurality ofphotosensitive cells arranged, each of the plurality of photosensitivecells generating a particular signal charge corresponding to an amountof incident light from the field, and an optical black area shieldedfrom said incident light, said optical black area being positioned at afront side and a rear side of horizontal scanning lines, the horizontalscanning lines comprising first horizontal scanning lines, includingfirst zones in which the photosensitive cells are present, and secondhorizontal scanning lines, including second zones in which thephotosensitive cells are absent, arranged alternately with each other atthe front side at predetermined intervals each; a driver for causingsaid solid-state image sensor to output an image signal in accordancewith exposure; an analog circuit for clamping the image signal outputfrom said solid-state image sensor; and a controller for controllingclamping executed by said analog circuit; said controller causing saidanalog circuit to selectively clamp the first horizontal scanning linesor the second horizontal scanning lines.
 4. The apparatus in accordancewith claim 3, wherein said controller causes said analog circuit toselectively clamp the first horizontal scanning lines or the secondhorizontal scanning lines in accordance with pickup information outputwhen picking up the field.
 5. The apparatus in accordance with claim 4,wherein said controller causes said analog circuit to prepare an imagefile in which information on the horizontal lines clamped is included inthe pickup information.